1. Technical Field
The present invention relates to a radio communication apparatus and a method of controlling transmission power.
2. Description of the Related Art
In an uplink for 3GPP LTE (3rd Generation Partnership Project Long Term Evolution, hereinafter referred to as LTE), estimation of the channel quality between a terminal (user equipment, UE) and a base station (BS or eNB) using a sounding reference signal (SRS) is supported. The SRS is used mainly for scheduling of an uplink data channel (physical uplink shared channel, PUSCH) (e.g., frequency resource assignment and selection of a modulation and coding scheme (MCS)). “Sounding” refers to estimation of the channel quality between a terminal and a base station.
In LTE, similar transmission power control (TPC) is performed for a PUSCH and an SRS. Specifically, transmission power of SRS (SRS transmission power) is determined by adding an offset to transmission power of a PUSCH (PUSCH transmission power). For example, in LTE, SRS transmission power PSRS(i) in subframe #i is determined by the following Equation 1.
[1]PSRS(i)=min{PCMAX,PSRS_OFFSET+10 log10(MSRS)+PO_PUSCH+α·PL+f(i)}  (Equation 1)
In Equation 1, PCMAX [dBm] denotes maximum transmission power of an SRS that can be transmitted from a terminal; PSRS_OFFSET [dBm] denotes an offset value for transmission power of a PUSCH to be transmitted from the terminal (parameter set by a base station); MSRS denotes the number of frequency resource blocks to be assigned to the SRS; PO_PUSCH [dBm] denotes the initial value of the PUSCH transmission power (parameter set by the base station); PL denotes a path loss level [dB] measured by the terminal; a denotes a weight coefficient indicating the compensation ratio of the path loss (PL) (parameter set by the base station); and f(i) denotes an accumulated value in subframe #i containing past TPC command (control values such as +3 dB, +1 dB, 0 dB, and −1 dB) in closed loop control.
Meanwhile, standardization of LTE-Advanced, which is a developed version of LTE, is started. In LTE-Advanced, support for uplink transmission in which a terminal uses a plurality of antennas (single user-multiple input multiple output, SU-MIMO) is being studied. The SU-MIMO is a technique in which a single terminal transmits data signals in a certain frequency at a certain time from a plurality of antennas to spatial-multiplex the data signals through a virtual communication path (stream) in a space.
In order to perform communication by SU-MIMO in LTE-Advanced, a base station must know the status of a propagation path between each antenna of a terminal and each antenna of the base station. Hence, the terminal must transmit an SRS to the base station from each antenna.
Regarding the uplink for LTE-Advanced, a technique is being studied in which common transmission power control is employed among a plurality of antennas of a terminal in order to control transmission power of a PUSCH and an SRS (for example, see NPL 1). Specifically, at the terminal, a single value is used as each parameter in the equation for determining SRS transmission power, which is shown as Equation 1, uniformly for all antennas. This can prevent an increase in signaling load required for transmission power control at a terminal having a plurality of antennas.